Follow these detailed steps carefully for perfect results. Patience, especially during the cooling stages, is key to achieving clean, beautiful bars.

Step 1: Prepare the Oven and Pan

Begin by preheating your oven to 350°F (175°C). Take a 9×13-inch baking pan and line it with parchment paper, leaving a 2-inch overhang on the two long sides. This parchment “sling” is essential for easily lifting the bars out of the pan for cutting later. Lightly grease the parchment paper and any exposed sides of the pan with butter or non-stick spray.

Step 2: Make the Hazelnut Shortbread Crust

In the bowl of a food processor, combine the 1 ½ cups of all-purpose flour, ¾ cup of finely ground hazelnuts, ½ cup of granulated sugar, and ¼ teaspoon of sea salt. Pulse a few times to evenly mix the dry ingredients. Add the cold, cubed ¾ cup of unsalted butter to the food processor. Pulse in short bursts, about 10-15 times, until the mixture resembles coarse, wet sand with some small, pea-sized butter pieces remaining. Be careful not to over-process; you don’t want it to form a solid ball of dough.

Step 3: Press and Bake the Crust

Transfer the crumbly mixture into your prepared 9×13-inch baking pan. Use your fingers or the flat bottom of a measuring cup to press the mixture firmly and evenly across the bottom of the pan, creating a solid, uniform layer. Bake in the preheated oven for 15-18 minutes, or until the edges are lightly golden brown and the crust looks set and dry. Once baked, remove the pan from the oven and set it on a wire rack to cool slightly while you prepare the filling. Do not turn off the oven.

Step 4: Prepare the Fudgy Mocha Filling

In a medium, heatproof bowl, combine the 1 cup of unsalted butter and the 8 ounces of chopped bittersweet chocolate. Create a double boiler by setting the bowl over a saucepan filled with about an inch of gently simmering water (ensure the bottom of the bowl does not touch the water). Stir the mixture occasionally with a silicone spatula until the butter and chocolate are completely melted, smooth, and glossy. Remove the bowl from the heat and immediately whisk in the 3 tablespoons of instant espresso powder until it is fully dissolved. Set the chocolate mixture aside to cool for about 5-10 minutes. It should be warm to the touch, not hot.

Step 5: Combine the Filling Ingredients

In a separate large mixing bowl, whisk together the 1 ½ cups of granulated sugar and the 4 room-temperature large eggs until the mixture is pale, slightly frothy, and well-combined, about 1-2 minutes. This incorporates a little air for a lighter fudginess. Whisk in the ½ cup of chocolate-hazelnut spread and the 1 teaspoon of vanilla extract until smooth.

Step 6: Finish the Batter

Gently pour the slightly cooled, melted chocolate-espresso mixture into the egg and sugar mixture. Whisk slowly until just combined. Be careful not to overmix at this stage. Finally, add the ½ cup of all-purpose flour and ¼ teaspoon of sea salt. Switch to a silicone spatula and fold the dry ingredients in until no streaks of flour remain. Again, do not overmix; stop as soon as everything is incorporated. Overmixing develops gluten and will result in a cakey, rather than fudgy, texture.

Step 7: Assemble and Bake the Bars

Pour the finished Fudgy Mocha Filling evenly over the pre-baked hazelnut crust in the pan. Use an offset spatula or the back of a spoon to spread the filling into a smooth, even layer, making sure it reaches all the corners. Place the pan back into the 350°F (175°C) oven and bake for 25-30 minutes. The bars are done when the edges are set and slightly puffed, and the center is mostly set but still has a very slight jiggle when the pan is gently shaken. A toothpick inserted into the center should come out with moist, fudgy crumbs attached, not wet batter. It’s better to slightly underbake than overbake to ensure maximum fudginess.

Step 8: Cool the Bars Completely

This is a critically important step. Remove the pan from the oven and place it on a wire cooling rack. Let the bars cool completely in the pan at room temperature, which will take at least 2 hours. For the best results and cleanest cuts, after they have cooled to room temperature, cover the pan with plastic wrap and transfer it to the refrigerator to chill for another 2-3 hours, or even overnight. Chilling solidifies the bars and makes the ganache application and cutting much easier.

Step 9: Make the Silky Chocolate Ganache

Once the bars are fully chilled, prepare the ganache. Place the 6 ounces of finely chopped semi-sweet chocolate in a medium heatproof bowl. In a small saucepan, heat the ⅔ cup of heavy whipping cream over medium heat until it just begins to simmer around the edges. Do not let it come to a rolling boil. Immediately pour the hot cream over the chopped chocolate. Let it sit, untouched, for 5 minutes. This allows the heat from the cream to gently melt the chocolate. After 5 minutes, whisk the mixture, starting from the center and working your way out, until the ganache is completely smooth, shiny, and emulsified.

Step 10: Top and Finish the Bars

Pour the warm ganache over the chilled bars in the pan. Use an offset spatula to spread it into a thin, even layer that covers the entire surface. Immediately sprinkle the ½ cup of toasted, chopped hazelnuts evenly over the ganache.

Step 11: Final Chill and Serve

Return the pan to the refrigerator for at least 30-60 minutes, or until the ganache is firm to the touch. Once the ganache is set, use the parchment paper overhangs to carefully lift the entire slab of bars out of the pan and onto a cutting board. For the cleanest cuts, use a large, sharp chef’s knife. Dip the knife in hot water and wipe it clean between each cut. Cut the slab into your desired size—24 small squares or 16 larger bars. The bars are best served at cool room temperature.